Stereophonic sound distributor



1952 R. H. RANGER STEREOPl-IONIC scum) DISTRIBUTOR Filed March 10, 19582 Sheets-Sheet 1 1 W 5m. \4%% flu hwmm @LNNQK Nov. 27, 1962 R. H. RANGERSTEREOPHONIC SOUND DISTRIBUTOR Filed March 10, 1958 2 Sheets-Sheet 2,Mgw I ITTOP/VEYS United States Patent 3,065,816 STEREOPHONIC SOUNDDISTRIBUTOR Richard H. Ranger, Newark, NJ.; Laura Lewis Ranger,

exeeutrix of said Richard H. Ranger, deceased, assignor t RaugertoneElectronics Corp., Newark, N.J., a corporation of New Jersey Filed Mar.10, 1958, Ser. No. 720,114 5 Claims. (Cl. 18131) This invention relatesto stereophonic sound distribution apparatus. The principles involved inits use are applicable to both the pick-up of sound for recordingstereophonically and for the projection of sound so recorded. Becauseits utilization in projection of previously recorded sound is generallyso much more widespread than use in the reciprocal manner, the inventionwill be described primarily as applied to reproduction. It will beunderstood, however, that because almost all sound reproducers can alsobe used as microphones for the pick-up of sounds, their characteristicsas regards directivity and frequency response being the same when usedin either fashion, explanations based upon the radiation pattern ofloud-speakers are equally applicable to the sensitivity patterns ofmicrophones.

By stereophonic sound as used herein, is meant sound so projected ordistributed that it appears to emanate from more than a single positionin space, e.g., so projected that when a speaking actor moves from oneside of a stage or screen to the other, or advances or retreats, hisvoice appears to move with him instead of its location being fixed at aposition of a single reproducer, or in the case of an orchestra, thesound of the string section appears to emanate from one side of thestage or room while that of the brasses appears to emanate from theother. It is now recognized that such stereophonic effects add grealy tothe illusion of presence. As the sense of direction from which soundappears to come is almost entirely due to the binaural eifectthedifference in phase or loudness of the sound as perceived by the twoears of the auditorto produce the true sterephonic effect requires, inthe case of recorded sound, at least two microphones, picking up soundsthat differ in loudness as between their various components, recordingsmade on two individual sound tracks and reproduction from at least twospaced loud-speakers. With this minimum of two complete sound channels,an auditor located substantially on the median line between the tworeproducers obtains an excellent stereophonic effect. If he moves morethan a short distance from either side of the median line, however, thepoint of origin of the sound appears to move with him toward the nearerof the two reproducers and it has been found that to get really goodsterephonic sound, the point of origin whereof appears to remainsubstantially constant when the auditor moves, has required at leastthree complete sound channels, using an intermediate reproducer locatedsubstantially midway between the other two.

Experiment has proved that the binaural effect or sense of directivityis derived almost entirely from the higher frequency components of soundwithin the audible range. The frequency at which it begins to becomeapparent is not sharply defined and depends to a degree upon theindividual characteristics of the auditor. In general, however, thesense of directivity becomes quite appreciable at frequencies in theneighborhood of 500 cycles per second. It increases gradually withincreasing frequency up to substantially the limit of theaudible range.

The sense of directivity is due to two cooperating factors. The first oftheseis differences in loudness of the sound as it reaches the two carsof the auditor; the second is difference in phase. Low pitch sounds oflong wavelength are readily detracted, i.e., they cast no sharp shadowsso that they are heard nearly as loudly in both ears, even though theymay proceed directly toward one side of the head of the listener.Further, the phase difference between the sound striking the two ears isvery slight in the case of low pitch, long wave sound. Moreover,ordinarily reproducing installations are mounted at the side of a roomwhich the listener faces, more or less as he would a stage, so that thepaths traversed by the sound in reaching his ears are not greatlydifferent in length and the phase difference is still further reduced.The difference in loudness is the more important of the two effects, butthey appear to work together up to frequencies where the wavelength ofthe sound becomes quite short in comparison with the distance betweenthe two cars, at which point the sound shadows become very sharp and thedifferences in phase become indeterminate.

The broad purpose of the present invention is to provide a sounddistributing system which takes advantage of these facts to produce thefull sterephonic effect utilizing two channels only, giving to anauditor in any part of an area in front of the projector the illusionthat the point of origin of the sound is at a definite fixed position inspite of the fact that he may be located nearer to a transducer on theopposite side of the equipment. Conversely, when used for pick-uppurposes the same type of structure will properly distribute the soundbetween two tracks on which it is to be recorded. Contributory to thisbroad purpose, among the objects of the invention are to provide a sounddistributing structure of minimum size which will give the effectsreferred to and which is applicable for installation in a living room ofordinary size; to provide a type of structure which can be made of suchdimensions as to give the stereophonic effect within a large theater; toprovide a sound distributing structure which can be made in unitary formand which is adaptable to the mounting of transducers of various types,in accordance with the tastes and preferences of a purchaser; and toprovide sound distributing structures which can be constructed inportable form to become an attractive piece of furniture forinstallation in any room and that does not require to be built in orrequire structural changes in the room wherein it is installed.

It is well known that electro-acoustic transducers always have somedegree of directivity. In general the degree of directivity depends uponthe frequency of the sound and the dimensions of the sound radiator orpickup, as the case may be, the radiator as here used being thediaphragm in transducers of the hornless type or the mouth of a hornwhere one is used. For sounds having Wavelengths that are long incomparison with the dimension of the radiator, the directivity ispractically nil, i.e., the polar response diagram is eithersubstantially spherical or if the radiator is mounted in a wall or otherbaffle, hemispherical. As the wavelengths become shorter than thedimension of the radiator directivity becomes more and more pronouncedas the ratio of radiator dimension to wavelength increases. Unlessspecial precautions are taken to avoid this effect the polar radiationdiagram of substantially any loud-speaker, plotted for frequencies inthe upper portion of the audible range, will have an elongated majorlobe, the axis whereof is perpendicular to the plane of the radiator.There may or may not be additional minor lobes located on either side ofthe major lobe, but these are not usually perceptible to a listener.Horn radiators are slightly less directive than open cones but thedifference is minor. In non-sterephonic sound distribution systems thisdirective characteristic has the result that auditors seated on ornearly on the axis of the sound projector will hear the full gamut ofthe reproduced sound, whereas those sitting materially to one side ofthe axis will hear the higher reproduced frequencies at de creasedrelative volume or lose them altogether. In such simple systems, thedirective properties of transducers are therefore a disadvantage; inaccordance with the present invention the directive properties areutilized to achieve the stereophonic effect.

In accordance with the present invention there is prcvided a sounddistributing structure comprising a sound distributing structurecomprising a sound reflecting surface in the form of a sector of acylinder having a vertical axis. The material of the cylinder may bemetal, wood, plastic or any other smooth, unyielding material and thetotal are subtended by the sector will, in general, be in the rangebetween a quadrant and an octant. On either side of the reflector thereare provided means for mounting one or more transducers, the responsepatterns whereof have an elongated major lobe for frequencies in theupper portion of the aiudible range. The mountings are so positionedthat the axes of the majorlobes intersect on the median line between thetwo transducers at or a short distance in front of the reflectingsurface, i.e., they intersect at an obtuse angle.

in the usual case, where the transducers are loudspeakers they may takeany of the conventional forms. Thus, for example, a single dynamic coneloud-speaker that is relied upon to reproduce all of the frequencieswithin the range of the apparatus has the necessary sound distributioncharacteristics as far as the high frequencies are concerned. in other,more elaborate systems, whereing woofers are used to reproduce the lowfrequencies and tweeters" to reproduce the higher ones, only the tweeterneed be mounted as described. In the latter case, however, it isadvisable that the cross-over point, at which the sound intensityradiated by woofers and tweeters is equal, should be in the neighborhoodof 500 cycles, where directivity becomes clearly perceptible; if thecross-over point is materially higher than this, the woofers also shouldpreferably be directed in the same manner as the high frequencyequipment.

The dimensions of the apparatus may vary widely, depending upon the sizeof the room in which the apparatus is to be used. For generalresidential use it has been found that a reflecting surface in theneighborhood of five feet wide and from two to two and a half feet highis suflicicnt to produce the stereophonic effect throughout the largestsizes of 1iving-room ordinarily encountered. In a large and deep theateror auditorium the reflecting surface can be extended acrosssubstantially the entire width of the room. In such large structures thecentral portions of the reflecting surface can be omitted, sincereflection does not occur in material degree from this portion of thestructure. The woofers can be placed at this point facing directlyoutward, with perceptibly interferring with the stereophonic effect.

The invention will be better understood by reference to the detaileddescriptions of certain embodiments of the invention which follows, thedescription being illustrated by the accompanying drawings wherein:

FIG. 1 is a plan view of a sound distributing system in accordance withthis present invention, embodiment in a cabinet or enclosure adapted forresidential use;

FIG. 2 is a front elevation of the distributor illustrated in FIG. 1;

FIG. 3 is a plan view of an auditorium showing the location of a sounddistributor in accordance with the inven-' tion The sound distributorillustrated in FIGS. 1 and 2 includes a cabinet or enclosure comprisinga base or floorboard 1, from which there rise corner posts or'struts 3that support a topboard or cover 5. Closing panels 7 and 9 may besupplied at the back and ends for dust-exclusion and appearance but theyare not operational elements of the invention. The major portion of thefront of the structure is closed by a convexly curved sound reflector11. In the equipment shown this is formed of plywood, bent into the arcof a cylinder having a vertical axis.

The structure is mounted upon three short legs includ ing a center leg13 at the rear and two front legs 15 at either side. Each of the frontlegs is carried up to approximately the level of the center of thereflecting surface to form the support for one edge of the horn 17 of ahigh frequency transducer or tweeter 29. The other edge of the horn issupported at the edge of the reflector 11. A hornless cone tweeter couldbe used mounted with the plane of support of the cone in the sameposition as the mouth of the horn.

in the apparatus shown, the space within the enclosure ehind thereflector is used to form folded horns for low frequency transducers orwoofers 21 of the dynamic, freecone type. As may be seen in FIG. 1, eachwoofer is mounted on a baffle 23, extending between the floorboard 1 andthe cover 5. The baffle 23 is positioned at a relatively sharp anglefacing a vertical septum 25 that divides the enclosure into twosymmetrical halves and forms, in effect, the throats of the folded hornswhich increase gradually in cross-section from the throat to the edge ofthe reflector, the latter thus becoming also one edge of the mouth ofthe born. The rear walls of each horn are formedby further barriers 27and 29, also extending from bottom to top of the enclosure and set atincreasingly large angles to the opposing face of the reflector thatforms its front wall. The barrier 29 terminates at the read of theenclosure where it is joined by a curved barrier 31 terminatingapproximately at the strut 3, to form the opposing edge of'the mouth ofthe horn. it will be noted that in this instance the dual horns thusformed face generally outward; the mouths of the horn forming theradiators have axes pointing generally outward and intersecting ratherfar in front of the reflector. In the particular apparatus describedhere this arrangement is dictated rather by mechanical convenience thanby stereophonic considerations since the woofers employed cut off at 500cycles and contribute little, to the stereophonie effect. Were thecut-off or crossover points of the woofers employed higher in frequency,it would be better to carry the horn mouths farther around so that theaxes of the radiation patterns would intersect more closely in front ofthe reflecting surface.

The important feature of the arrangement is the mounting of thetweeters. Those illustrated have elliptical horns, mounted with themajor axes of the ellipses in a horizontal palne. Their radiationpatterns are therefore much more directive inthe horizontal plane thanthey are in the vertical one (the radiation pattern of one tweeter 57being shown at 61 in FIGURE 3, with an elongated major lobe 63). This isdesirable as it makes the sound distribution better vertically and morenearly the same for seated or standing auditors or for listeners ofdifferent heights. The axes of the radiation patterns of the tweetersare indicated by the dot-dash lines 33. It will be seen that these axesintersect in the median plane between the two tweeters, a few inches infront of the surface of the reflector. For purposes of illustration, andnot by way of limitation, it may be stated that the total chord width ofthe reflector 11 in the apparatus described is 63 inches, and the pointof intersection of the axes of the radiation pattern is about 2 inchesin front of the center of the reflector. The total are subtended by thereflector is about 70, its radius of curvature being approximately 45inches. None of these dimensions is critical. A larger arc,corresponding to a shorter radius of curvature of the same width ofreflector, results in a deeper zone wherein the stereophonic' effect isapparent, a smaller arc makes the zone shallower; on the other hand, ashorter radius of curvature tends to narrow the zone and vice versa.

The positioning of the transducers, particularly as regards to the angleat which they are set, depends in part on the shape of their radiationpatterns in the upper region of the audible range. The elongation of themajor lobe of these patterns does not begin to become apparent until thesize of the radiator exceeds a half wavelength of the radiated sounds.For the purpose of analysis the origin of the sound can be considered asthe center of the radiating area. It is therefore clear that the axis ofthe radiator must be displaced outwardly from the edge of the reflector.Reflection of sound follows the same rules as that of light, in that theangles of incidence and reflection are equal. Considering, forsimplicity, that the source of the radiated sound is the center of thehorn mouth, it will be seen that the reflection from the surface of theelement 11 close to the mouth of the horn will be transverse to the axisof the distribution pattern, While that striking the reflector fartherand farther from the horn will become closer and closer to the axisuntil the point of tangency is reached and no reflection will occur.

The effect of this is to increase the apparent loudness of the soundfrom the horn to an auditor positioned in front of the distributor andon the same side as the horn. Furthermore, it scatters the beam of soundmore widely with respect to people on the opposite side of theequipment. The inverse square law, as to relative intensity of sound atvarying distances from the mouth of the horn, still obtains, as long asone considers the actual path traversed by the sound and measures thedistance along a single sound ray.

A pair of crossed directional reproducers, mounted as here shown anddescribed, but without the intervening reflecting surface, will give thestereophonic effect to auditors Within a very limited zone centered onthe axes of the major lobes of their response diagrams, just as will asingle pair of transducers facing outward toward the auditors but likethe latter arrangement the zone is highly restricted. The present deviceboth widens and deepens the zone within which the effect is stronglyapparent.

It should be realized that the stereophonic effect is an auditoryillusion having subjective as well as objective aspects. Both aspectsare important. Objectively, the effect will vary to some extent with theradiation pattern of the transducers used, as well as with the size ofthe structure in comparison with the size of the room in which it is tobe used, the degree of convexity of the reflecting surface and the anglebetween the axes of the transducers at their point of intersection. Itis this latter factor that is most important, although there is aconsiderable degree of latitude even as to this parameter. It has beenfound that the planes 65 (FIGURE 3) of the sound radiators-the mouths ofthe horns or the mountings of the cone diaphragms-should make a sharperangle at their point of intersection than do the radii of the reflectordrawn from its reflective edges, where it intersects the planes of theradiators. The axes of the transducers should therefore come very nearlytangent to the surface of the reflector, as shown in the drawing. Once atype of transducer has been determined upon and mounted in approximatelythe relationship shown in the drawing, very little experiment willindicate the angle giving the widest zone wherein the stereophoniceffect is apparent. In general, too obtuse an angle of intersectionbetween the axes of the transducers narrows the zone of stereophonicperception less than does too acute an angle. Thus, if the twotransducers are set with their planes more nearly parallel so that theiraxes become tangent to the surface of the reflector or even intersect inthe reflector surface on the median line, good stereophonic results canstill be obtained throughout nearly as wide a zone as if set as shown,whereas if the transducers are set so that their radiating planes are onradii of the reflector, the latter has little eifect and the zone or"stereophonic elfect becomes nearly as limited as if the reflector wereabsent. The difierences between transducers are, however, minor and thegeneral arrangement shown has been proved to be satisfactory even withrelatively large differences in scale. What may be called thestereophonic zone occupies an area extending lat- "3 erally two to threetimes the length of the structure and somewhat more than this distancedirectly in front of it. With the dimensions here shown this issuflicient to fill even a very large living room.

In the experimental determination of the best arrangement for a givensize and curvature of reflector the same signal, preferably of mixedfrequency, is supplied to both high-frequency transducers. The resultingsound should appear to come from the mid-point of the structure to anobserver at a distance of from one to three times the separation of thetransducers in front of the device, and at a distance to one side of itsmedian line at least as great as that of the transducers. If thetransducer axes intersect at too obtuse an angle the sound will appearto come from the farther of the two; if the angle is too acute itappears to come from the nearer one. The range of adjustment for anycurvature of reflector is quickly and easily found and the stereophonicetfect, for sounds of unequal intensity from the two transducersfollows.

The stereophonic eifect becomes gradually less apparent, of course, asthe distance from the sound distributor increases. At extreme distanceswhere the total angle subtended by the entire structure becomes only afew degrees, the differences in both phase and loudness of the sounds asthey strike the two ears of the auditor become very slight indeed andthe whole arrangement becomes, in effect, a point source of sound. Atnormal listening distances, however, the arrangement has the property ofapparently separating the sound sources more widely than the transducersthemselves are separated.

It follows from What has been stated above that a relatively smalldevice such as has been described in connection with FIGS. 1 and 2 isnot suitable for a large sized auditorium or theater. To increase boththe width and the depth of the stereophonic zone so as to fill a theateran arrangement such as is shown in purely diagrammatic form in FIG. 3can be adopted. This figure shows the plan of a typical motion picturetheater, indicating the position of the stage 41, the plane of thescreen 53 and the curved apron 45 projecting forward from the stage.

In many theaters the edge of the apron is substantially the curvaturebest adapted to the present invention, which makes it relatively easy toinstall a curved reflector following its curvature. The high frequencytransducers 47 can be mounted on either side of the apron at the base ofthe reflector 49, preferably pointing somewhat upward to take advantageof the full height of the reflecting surface, it being noted thatrelatively little directive effect is observable in the vertical planeand the bottom of the reflecting surface will in some instances be in anorchestra pit below the level of all of the auditors in the theater. Tothe extent that the high frequency transducers are directive in thevertical plane their upward slant makes substantially no difference asto directivity as this affects stereophonic reception but does preventundesired reflections from the opposite side of the orchestra pit or anyother nearby objects.

In this instance the reflector 49 is interrupted in the center beyondthe point at which the transducer axes approach it most closely, leavinga gap between their edges, the points indicated bythe bracket 51. Thewoofers can be mounted in this gap if desired, or they can be mountedabove and either side of the screen without destroying the stereophoniceffect, provided the cutoif is sufliciently low, not above approximately500 cycles. Alternatively, they can be mounted in the same position asshown for the tweeters 47 if they radiate any material energy in theupper portion of the band of audibility.

It should be evident that the invention may be embodied in numerous waysother than the two typical ones described. For example, in motionpicture theater constructed without the typical stage apron or orchestrapit of theaters intended for stage productions, an entirely separatereflecting structure can be installed below or even above the screen. Atthe other end of the dimensional scale more compactstructures than thathere illustrated can be used; a considerably portion of the width of theequipment shown in FIGS. 1 and 2 is taken up by the mouths of the wooferhorns. If, for example, a single cone-type transducer is employed ineach channel instead of the high and low frequency units illustrated,these units being mounted in the positions occupied by the tweeters inthe figures, the length of the equipment will be shortened materially,since in this case it is only necessary to provide enough space behindthe transducers to baffle and absorb the waves from the back of thecones without setting up unpleasant resonances.

The greater the width of the reflector, the larger the area in which thestereophonic effect is apparent. As the effect is an illusion the senseof position or apparent position of the sound source as reported to theear of the listener will be rejected if strongly negatived by his othersenses; he may sense that the sound to which he is listening emanatesfrom either side of the wall of the room which he faces, but he willtend to reject the idea that it comes from more Widely separated pointsthan these. auditors. Tests indicate that most subjects, listening withclosed eyes, obtain a sense of source position extending beyond thelimits of the curved reflector; therefore, in a small room reflectornarrower than that here describedapproximately five feet-will givefairly satisfactory results. The minimum width that will give asatisfactory illusion to most subjects appears to be in the neighborhoodof three feet but because the effect is subjective and varies withdifferent subjects no definite limits can be set.

The directive properties of both binaural hearing and transducerresponse or radiation are most apparent in the upper portion of theaudible range, as has already been pointed out. Very few sounds are puretones, however, and most sounds recognized as being in the low frequencyrange are accompanied by harmonics or nonharmonic overtones in theupper, directive range. A listener in most cases recognizes thecomposite sounds as single entities, and will assign their directionrelative to himself as that from which the high frequency componentsappear to come. Therefore, although it is prefer:

able that the transducers used begin to show .directivity atapproximately 500 cycles this frequency has been arbitrarily consideredherein as the approximate lower limit of the upper audible range, verymarked effects, quite satisfactory to nearly all auditor s, can beachieved even if the elongation of the major lobe of the responsepattern of the transducers used does not become pronounced until thefrequencies radiated become considerably higher than this.

For these various reasons the examples heregiven are not intended tolimit the scope of the invention, but rather to serve as illustrationsof typical. applications thereof. All intended limitations on the scopeof the invention are set forth in the claims. 7

What is claimed is as follows: 7

1. A stereophonic sound distributor comprising a sound The effect issubjective and differs as between reflecting member havingaconvex'reflecting surface dis, posed as the arc of a cylinder about avertical axis, and a pair of similar electro-acoustic transducersmounted substantially symmetrically at opposite sides of said sur-vface, said transducers each having a directional response patternincluding an elongated major lobe and being so mounted that the axes ofsaid major lobes intersect substantially in the medium plane betweensaid transducers in front of said reflecting surface.

2. A stereophonic sound distributor comprising a sound reflecting memberhaving a convex reflecting surface disposed as the are a cylinder havinga vertical axis and subtending an angle within the range between 450 and90, and a pair of similar electro-acoustic transducers mountedsubstantially symmetrically at opposite sides of said surface, saidtransducers each having a directional response pattern including anelongated major lobe and being so mounted that the axes of said majorlobes intersect substantially on the medium plane between saidtransducers and in front of said convex surface.

3. A stereophonic sound distributor comprising a sound reflecting memberhaving a generally convex sur: face, and means for mounting a pairof'loud speakers symmetrically at opposite sides of said surface withone loudspeaker of the said pair thereof located at each side of saidsurface so that the axes defined byperpendiculars. extending from thecenters of the radiators of such loudspeakers intersect at an obtuseangle substantially on the median plane between said loudspeakers andproximate the said generally convex surface.

4. A stereophonic sound distributor comprising a sound reflecting memberhaving a convex reflecting surface disposed as the sector of a cylinderhaving a vertical axis and subtending an arc in the range between 45 and90, and means disposed symmetrically at opposite sides of said soundreflecting surface for supporting a pair of loudspeakers whereby oneloudspeaker of the said pair thereof is at each side of the reflectingsurface so that the planes of the radiators thereof intersect behindsaid surface at a smaller angle than that subtended by it and that theaxes of said radiators as defined by perpendiculars directed from thecenters thereof intersect in front of said convex surface in front ofits median plane.

5. A sound distributor as defined in claim 4 wherein the ange subtendedby said sound reflecting member is in the range between and ReferencesCited in the file of this patent UNITED STATES PATENTS Great BritainOct. 1, 1951

